1. Field of the Invention
The present invention relates to a method for forming a photo diode on a semiconductor wafer, and more particularly, to a method for simultaneously forming a photo diode and a CMOS transistor on a semiconductor wafer.
2. Description of the Prior Art
The photo diode is a semiconductor device comprising a photo-conductivity cell and a junction diode, and is commonly applied to manufacture photoelectric products, such as cameras and photo sensors of scanners. The light current of the photo diode represents signals, the dark current represents noises, and the photo diode processes signal data by using the intensity of the signal to noise ratio. In the semiconductor industry, it is often to increase the light current of the photo diode so as to increase the signal to noise ratio, and hence to enhance the contrast of the signal. As a result, the sensitivity of the photo diode is enhanced and the quality of the photo diode is improved.
Please refer to FIG. 1, FIG. 1 is a schematic diagram of a photo diode 17 according to the prior art. A semiconductor wafer 10 comprises a substrate 14, a P-type well 16 positioned on the substrate 14, a NMOS transistor 11 and a photo sensing area 12 formed on the P-type well 16, and a field oxide 18 positioned on the substrate 14 and surrounding the NMOS transistor 11 and the photo sensing area 12. The field oxide 18 forms a electrical isolation to prevent short circuits.
The method for forming the photo diode 17 according to the prior art uses arsenic (As) atoms at a high dosage as the dopant to proceed an ion implantation process. The energy and the dosage for the ion implantation process is 80 KeV and 10.sup.15 cm.sup.-2, respectively. The ion implantation process forms a N-type doped area 13 on the surface of the P-type well 16. A depletion region 15 for detecting the light current is formed across the PN junction between the doped area 13 and the adjacent P-type well 16.
In the formation of the photo diode 17 according to the prior art, As atoms at high dosage for forming the source and the drain of the MOS transistor are used as the major dopant to simultaneously form the doped area 13 on the surface of the P-type well 16. The crystal structure on the surface of the photo sensing area 12 may be damaged during the ion implantation with high dosage ions. Thus, in the PN junction of the photo diode 17 more dark current is generated, resulting in more noises for the photo diode 17. Also, the doped area 13, which is formed by the ion implantation with high dosage ions, has a depletion region 15 with a narrow width, leading to a decrease in the real active region of the sensing area 12. Therefore, when the photo diode 17 is irradiated by lights, the light current sensed by the depletion region 15 is reduced and the signal to noise ratio is lowered. And hence, the sensitivity of the photo diode 17 is reduced.
In addition, the higher implantation energy also causes a deeper PN junction depth. When the photo diode 17 is irradiated by short wavelength lights, the blue light for instance, the light current induced by the PN junction of the photo diode 17 becomes smaller, because the short wavelength light has a shallow penetration depth in the silicon wafer. Consequently, the sensitivity of the photo diode 17 for detecting short wavelength lights is reduced.